The IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IP Multimedia services provide a dynamic combination of voice, video, messaging, data, etc. within the same session. The IMS is defined in the 3GPP Specification 23.228.
The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly.
FIG. 1 illustrates schematically how the IMS 3 fits into the mobile network architecture in the case of a GPRS/PS access network. Although numerous network entities, or nodes are depicted, only those relevant to the present discussion have been assigned reference numerals. As shown in FIG. 1 control of communications occurs at three layers (or planes). The lowest layer is the Connectivity Layer 1, also referred to as the bearer, or traffic plane and through which signals are directed to/from user terminals accessing the network. Access to the IMS 3 by IMS subscribers is performed through an IP-Connectivity Access Network (IP-CAN). In FIG. 1 the IP-CAN is a GPRS network including entities linking the user equipment to the IMS 3 via the connectivity layer 1. The GPRS network includes various GPRS Support Nodes (GSNs).
The IMS 3 includes a core network 3a, which operates over the Control Layer 4 and the Connectivity Layer 1, and a Service Network 3b. The IMS core network 3a includes various network nodes that include Call/Session Control Functions (CSCFs) 5. The CSCFs 5 include Serving CSCFs (S-CSCF) and Proxy CSCFs (P-CSCF), which operate as SIP proxies within the IMS in the middle, Control Layer 4. Other IMS core network entities shown include a Media Resource Function Controller (MRFC), a Border Gateway Control Function BGCF and a Media Gateway Control Function, (MGCF) 5a. The IMS also includes a Home Subscriber Server (HSS) 6, which supports the IMS nodes that handle calls and performs authentication and authorization of the user. The HSS 6 may include or share access of data from a Home Location Register (HLR—not shown), which is a master user database that contains subscription-related information (subscriber profiles).
At the top is the Application Layer 7, which includes the IMS service network 3b. Application Servers (ASs) 7a are provided for implementing IMS service functionality. Application Servers 7a provide services to end-users on a session-by-session basis, and may be connected as an end-point to a single user, or “linked in” to a session between two or more users. Certain Application Servers 7a will perform actions dependent upon subscriber identities (either the called or calling subscriber, whichever is “owned” by the network controlling the Application Server 7a).
The IMS relies on Internet Protocol (IP) as a transport technology. Using IP for voice communications, however, presents some challenges, especially in the mobile community where Voice Over IP (VoIP) enabled packet switched (PS) bearers may not always be available. To allow operators to start offering IMS-based services while voice enabled PS-bearers are being built out, the industry has developed solutions that use existing Circuit Switched (CS) networks to access IMS services. These solutions are referred to as IMS Centralized Services (ICS). ICS is described in 3GPP TS 23.292 (with further aspects described in 3GPP TS 24.292 and 3GPP TS 29.292) and is also the name of the Work Item in 3GPP Release 8 addressing these matters. ICS allows a User Equipment (UE) to connect to a CS access network and to have access to Multimedia Telephony services. ICS allows for the delivery of consistent IMS services to the user regardless of the attached access type (e.g. CS domain access or IP-CAN).
FIG. 1 also shows a Circuit Switched (CS) domain 8. A call from a User Equipment (UE) is routed by a Mobile Switching Centre (MSC) 8a. The MSC where a subscriber is currently located is referred to as the visited MSC (V-MSC), while the Gateway MSC (G-MSC) 8b is the MSC that determines which MSC is the V-MSC that currently serves the subscriber who is being called. The V-MSC has an associated Visitor Location Register (VLR) which is a database of subscriber data for the subscribers currently being served by the V-MSC.
Referring to FIG. 2, where the equivalent entities have the same reference numerals as FIG. 1, an ICS-enabled UE 9 can access an MSC Server 8a via a CS Access network 10. It also accesses a CSCF 5 via a Gm reference point, and a Service Centralization and Continuity Application Server (SCC AS) 11 via a Gm reference point. SIP is used to perform service control between the ICS UE 9 and the SCC AS 11 over the Gm interface. For a speech service, the ICS UE 9 can use its CS access to transfer voice media. The ICS procedures include mechanisms whereby the MSC Server 9 is enhanced for ICS so that it can communicate directly with the IMS (e.g. CSCF 5) via the 12 interface. This is used, for example, for call origination, call termination and registration.
The SCC AS 11 is a home network based IMS Application Server that provides the functionality required to enable IMS Centralized Services. The SCC AS 11 is inserted in the session path using originating and terminating initial Filter Criteria (iFC); it is configured as the first AS in the originating iFC chain and as the last AS in the terminating iFC chain The SCC AS 11 may also be invoked through the use of Public Service Identifier (PSI) termination procedures when using CS access.
An incoming call for an ICS subscriber with a service provided by the IMS can be received either through the CS domain or via the IMS. Some, or all calls for an ICS subscriber that are received through the CS domain need to be routed to the IMS for service execution, prior to onward routing of the call to the subscriber. This is sometimes referred to as Terminating Service Domain Selection (T-SDS), or more informally to establish a terminating leg. Although the 3GPP ICS specifications do not stipulate any specific procedures, an informative annex in 3GPP TS 23.292 release 10 (Annex F.2) includes a number of procedures based on techniques available in the current CS networks. The following 5 procedures (reproduced in italics below) have been extracted from that annex. However, each of the 5 procedures has drawbacks, as explained after each one below.
1. Use of CAMEL for Call Diversion to IMS
This option applies to configurations requiring handling of incoming calls at the GMSC function. Upon receipt of an incoming call, the GMSC queries the HSS for routing information via the Send Routing Information (SRI) query. The user profile in the HSS is configured to return T-CSI including a gsmSCF address to the GMSC in response to the SRI query. When handling calls for a subscriber with a service provided by the IMS, the subsequent processing at the gsmSCF and the GMSC results in routing of the call to the IMS using the IMRN. The call is routed to the SCC AS according to standard IMS routing procedures. In order to determine the necessary information to complete the call, the SCC AS uses the IMRN or the ISUP information mapped to SIP headers.
CAMEL is short for Customised Applications for Mobile networks Enhanced Logic (see ETSI TS 123 078). In the extract above T-CSI is short for Terminating CAMEL Subscription Information; gsmSCF is short for GSM Service Control Function; IMRN is an IP Multimedia Routing Number; and ISUP is short for ISDN User Part. Use of CAMEL for call diversion to IMS, requires provisioning of CAMEL trigger information in subscriber data as well as configuration of associated routing data. It also involves an overhead in the form of additional call signalling of a CAMEL trigger invocation and (as described in the Annex) a PSI-routed leg within IMS from the MGCF 5a (see FIG. 1) to the SCC AS 11.
2. HSS Directed Call Diversion to IMS
This option also applies to configurations requiring handling of incoming calls at the GMSC function. Upon receipt of an incoming call, the GMSC queries the HSS for routing information using the MAP Send Routing Information (SRI) procedure (as defined in TS 29.002). Based on a non-standardized mechanism, the user profile in the HSS is configured to return an IP Multimedia Routing Number (IMRN) to the GMSC in response to the SRI query, when the call is directed to a subscriber with a service provided by the IMS. The subsequent processing at the GMSC results in routing of the call to IMS using the IMRN. Two methods can then be used to ensure correlation between the IMRN and the original called party.                a Cooperative allocation/deallocation: In this method, the IMS is made aware of the assigned IMRN and when a call is received for that number, the original number is retrieved. This method is similar to the Provide Roaming Number procedure in MAP (see TS 29.002).        b Algorithmic: In this method, a known algorithm is used to derive the IMRN at the CS [i.e. in the CS network], and to deduce the original called number from the IMRN at the IMS. One method of performing such an algorithm could be use of a prefix.        
MAP is short for the Signaling System 7 protocol Mobile Application Part. HSS-directed call diversion to IMS is based on a non-standardized mechanism used to configure the user profile in the HSS to return an IMRN to the GMSC in response to the SRI query. It also requires specific routing data configuration to handle the routing to IMS using the IMRN. Sub-option a. requires that the IMS is made aware of the IMRN and is able to replace the IMRN with the original called number when the call reaches the IMS, but it no procedures to handle this are described.
3. Static Diversion from GMSC with Dedicated Trunk Groups
This option also applies to configurations requiring handling of incoming calls at the GMSC function. Dedicated trunk groups can be used at the GMSC to divert CS terminations to the MGCF.
The drawback with this procedure is that it requires dedicated trunk groups to be configured and managed.
4. Static Diversion Using Local Number Portability
This option can be used for routing of calls originating in PSTN networks to IMS. A Local Number Portability database dip can be used to reroute incoming calls to a subscriber with a service provided by the IMS with calls to the MGCF.
In the extract above PSTN stands for Public Switched Telephone Network. The drawback with this procedure is that it requires porting procedures to be used when enabling ICS for a subscriber, which may impact on the operator's Business Support System and incur interruption in service delivery to the user.
5. Direct Routing to IMS
Translations can be set up in the PSTN network to route the incoming call to a subscriber with a service provided by the IMS to the MGCF. This way the normal IMS routing technique specified in TS 23.228 can be used.
The drawback with this procedure is that it requires specific number series to be used for ICS subscribers.